Enhancement of aerosols near the cold tropopause in summer over Tibetan Plateau: lidar and balloonborne measurements in 1999 at Lhasa, Tibet, China

Abstract

Tropospheric and stratospheric aerosols were measured with a lidar and a balloon-borne optical particle counter at Lhasa (30 N, 91 E), Tibet, China in August, September, and October, 1999. Also, the vertical profile of ozone by ozone sonde measurements were gained in same period. Ozone concentrations of this area were smaller than those of other region of same latitude especially near tropopause. The number concentration of particles (diameter larger than 0.3 tm) showed noticeable enhancement near the local tropopause, and temporal changes in the particle concentration of the enhanced layer, especially in tropospheric part of the layer, seemed to correlate well to the changes in water vapor content in the mid, and possibly upper, troposphere. Those observations suggest the possibility that the tropopause aerosol particle enhancement associates with water vapor transportation from the lower troposphere to near the tropopause which is caused by active upwelling motion of air over the Tibetan plateau during Asian summer monsoon period. In other words form of aerosols near the tropopause possibly affected by monsoon activities in summer over the Tibetan plateau. The temperature of the local tropopause observed at Lhasa, Tibet was very cold and became frequently lower than frost point of nitric acid trihydrates (NAT) during observational period, and that atmospheric condition contributed to activation of gas to particle conversion process near the tropopause in summer over the Tibetan plateau. The lidar measurements suggested that the enhanced particle (scattering ratio=1.25) had low depolarization ratio, c =0.5%, and showed possibility that particulate near the tropopause was composed of liquid phase particles and not solid state such as cirrus cloud particles. Those lidar measurements, however, were not in the same range of polar stratospheric particles (PSCs) type Ia and/or lb given by previous lidar measurements at polar regions suggesting necessary of new particle formation scenario, which is different from PSCs formation processes in the poiar winter stratosphere, near the cold summer tropopause over the Tibetan plateau.